With modern methods of medical diagnosis, such as for example X-ray computed tomography (CT), image data of an examined object of measurement can be obtained. The examined object of measurement is generally a patient.
X-ray computed tomography—referred to hereafter as CT for short—is a special method of radiography which differs in principle from the classic method of tomography in the construction of the image. In the case of CT scans, transversal sectional images are obtained, that is to say images of sections of a body, which are oriented substantially perpendicular to the axis of the body. The tissue-specific physical variable represented in the image is the distribution of the attenuation of X-radiation μ(x,y) in the sectional plane. The CT image is obtained by a reconstruction of the one-dimensional projections, supplied by the measuring system used, of the two-dimensional distribution of μ(x,y) from numerous different viewing angles.
The projection data are determined from the intensity I of an X-ray beam after it has passed through the layer of which an image is to be formed and its original intensity I0 at the X-ray source according to the law of absorption
                              ln          ⁢                                          ⁢                      I                          I              0                                      =                  -                                    ∫              L                        ⁢                                          μ                ⁡                                  (                                      x                    ,                    y                                    )                                            ⁢                              ⅆ                l                                                                        (        1        )            
The path of integration L represents the path of the X-ray beam under consideration through the two-dimensional attenuation distribution μ(x,y). An image projection is then made up from the measured values of the line integrals through the object layer, obtained with the X-rays of a viewing direction.
The projections—characterized by the projection angle α—originating from many different directions are obtained by a combined X-ray tube detector system, which rotates around the object in the plane of the layers. The devices which are most commonly used at present are known as “fan beam devices”, in which a tube and an array of detectors (a linear or part-circular arrangement of detectors) rotate together in the plane of the layers about a center of rotation, which is also the center of the circular measurement field. The “parallel beam devices”, which have very long measuring times, are not explained here. However, it should be pointed out that a transformation from fan projections to parallel projections and vice versa is possible, so that the present invention, which is to be explained on the basis of a fan beam device, can also be used for parallel beam devices without any restriction.
In FIG. 1, a computed tomography device for a fan beam method is schematically represented. In the case of this device, X-ray tubes 1 and beam receivers 2 (detectors) rotate together about a center of rotation, which is also the center of the circular measurement field 5, and in which the patient 3 to be examined is lying on a patient bench 4. To allow different parallel planes of the patient 3 to be examined, the patient bench can be displaced along the longitudinal axis of the body. The advancement of the patient bench is generally referred to as “pitch”.
As can be seen from the drawing, CT scans produce transversal sectional images, that is images of layers of the body which are oriented substantially perpendicular to the axis of the body. This method of layered representation presents the distribution of the attenuation value μz(x,y) itself (z is the position on the longitudinal axis of the body).
Computed tomography (referred to hereafter as CT) requires projections from very many angles α. To generate a tomogram, the cone of rays emitted by the X-ray tube 1 is masked in such a way as to produce a planar fan of rays, which casts one-dimensional central projections of the radiographed layer. For the exact reconstruction of the distribution of the attenuation values μz(x,y), this fan of rays must be perpendicular to the axis of rotation and also spread so wide that it completely covers the layer aimed at of the object of measurement from every direction of projection α. This fan of rays passing through the object is picked up by detectors, which are arranged linearly on a segment of a circle. In the case of commercially available devices, there are up to 1000 detectors. The individual detector reacts to the incident rays with electrical signals, the amplitude of which is proportional to the intensity of these rays.
Each individual detector signal belonging to a projection α is in each case picked up by measuring electronics 7 and passed on to a computer (system computer) 8. With the computer 8, the measured data can then be processed in a suitable way and visually displayed on a monitor 6, initially in the form of a sinogram (in which the projection α is plotted as a function of the measured values of the corresponding channel β) in what are known as Gordon units, but finally in the form of a natural X-ray image in Hounsfield units.
According to the prior art, the person operating the CT device (generally the doctor) must perform settings on the CT device in order to achieve the desired image quality, which, inter alia, is also characterized by the contrast or the signal-to-noise ratio.
At the present point in time, these setting possibilities are very elementary. Current intensity, voltage, region of the patient to be recorded, layer thickness, pitch, etc., can be set. It is also possible to set whether the CT device is operated in spring focus or with a high-resolution comb. Both operating modes are explained in more detail later.
The disadvantage here is that the frequency response of the measuring electronics cannot be adapted to the chosen settings, as a result of which the entire informational content of the CT signal cannot be interpreted. In order nevertheless to obtain approximately the desired image quality, the signals obtained are processed after measurement by additional low-pass or high-pass filters. As stated, this is only possible in an approximate manner, since the electronics available for subsequent processing are predetermined by hardware and do not allow any latitude.